1. Field of the Invention
The present invention relates to the field of electrical machines, in particular generators having amortisseur bars.
2. Discussion of Background
Amortisseur windings ("amortisseur bars") are used in the rotors of electrical machines for various reasons. The arrangement and function of such amortisseur bars are explained, for example, in publications U.S. Pat. No. 4,453,101, U.S. Pat. No. 5,122,698 or U.S. Pat. No. 5,606,211. In this case, the amortisseur bars may be arranged in the winding slots between the winding bars and the locking wedges, as shown in U.S. Pat. No. 5,122,698. However, they may also be designed as locking wedges of winding slots, as shown in U.S. Pat. No. 5,606,211. The amortisseur bars are electrically connected to one another at their ends by amortisseur rings or amortisseur segments slipped over the rotor.
A specific use of the amortisseur bars relates to the pole zones of rotors. In the pole zones, in which there are no winding bars, special slots having a small depth are provided for accommodating the amortisseur bars, which are inserted or pushed into said slots (U.S. Pat. No. 4,363,986). So that the amortisseur bars are securely held in the slots by the centrifugal forces occurring at the high rotational speeds during operation, special provisions have to be made. This may be achieved by locking wedges pushed into the slots above the amortisseur bars (U.S. Pat. No. 5,122,698). However, this may also be achieved by retaining rings shrunk onto the rotor (U.S. Pat. No. 4,363,986). Finally, however, it is also possible to design the amortisseur bars themselves like locking wedges (U.S. Pat. No. 5,606,211).
In all cases, it has to be taken into account that the amortisseur bars must be pushed over a relatively large rotor length into the slots and secured there. In order not to make it unnecessarily difficult to push in the amortisseur bars during fitting, sufficient clearance is provided between the amortisseur bars or locking wedges and the slots. On the one hand, the result of this is that the amortisseur bars, at low rotational speeds or when the machine is stopped, only rest loosely in the slots provided for this purpose and tend to rattle. On the other hand, however, the result of this, in particular, is also that, at low rotational speeds or when the machine is stopped, the electrical contact between the ends of the amortisseur bars and the amortisseur rings or segments above them, but also between the amortisseur bars and the rotor core, either does not exist at all or is inadequate on account of the lack of contact pressure, so that scorching may occur when the amortisseur bars are carrying current. At the high rotational speeds during normal operation, however, the amortisseur bars and locking wedges are pressed outward by the centrifugal forces produced and are fixed by the corresponding wedge shape of the slots, so that the play is removed and at the same time sufficient contact with the amortisseur rings or segments and the rotor core is produced and ensured.
In order to avoid the adverse effects of the mechanical play when the machine is stopped or at low rotational speeds of the machine, it has been proposed in U.S. Pat. No. 5,122,698 to insert spring strips curved in the transverse direction into the slot below the amortisseur bars, which spring strips press the amortisseur bar (and the locking wedge above it) against the bevels in the side walls of the slot and at the same time press the projecting ends of the amortisseur bars against the amortisseur ring. However, even during fitting, the inserted spring strips result in very high frictional forces when the locking wedge is pushed in, so that in this solution a special lever mechanism has to be used for the pushing-in (FIG. 6 in U.S. Pat. No. 5,122,698) and ease of fitting by hand is no longer possible.